FIG. 1a illustrates a closed container 10 with a small discharge orifice 14 containing liquid 11. The pressure of the air 12 in the container 10 is equal to atmospheric pressure Po. The pressure in the liquid 11 at any depth increases from the value of the air pressure at the surface level 13 of the liquid 11 by an amount equal to the density of the liquid times the vertical distance below the surface 13 of the liquid 11. This pressure distribution is illustrated in FIG. 2 for the container 10 when the container 10 is inverted and the orifice 14 is blocked. If the container 10 is inverted and if the liquid 11 is water, an orifice diameter of less than 0.2 inches will prevent atmospheric air from entering the container 10 through the orifice 14. Since the pressure of the liquid 11 increases by an amount equal to the density (d) of the liquid 11 times the depth below the surface 13, it should be noted that the difference in the liquid pressure from the depth h1 to the depth h2 is equal to the product of the density times the vertical distance between h1 and h2 and is independent of the pressure of the air (Pair) or the amount of liquid 11 in the container 10.
FIG. 1b illustrates the container 10 when it is inverted and the small discharge orifice 14 is located a vertical distance H1 below the surface 13 of the liquid 11. Since the air pressure in the container 10 is at atmospheric Po, the pressure of the liquid at the discharge orifice 14 is greater than the atmospheric pressure by the product of the liquid density (d) times the vertical distance (H1) below the surface 13 of the liquid. That is, P=Po+dH1, where P is the pressure on the container-side of the discharge orifice 14. This added pressure causes liquid 11 to flow out of the container 10 to the atmosphere through the discharge orifice 14. As liquid 11 flows out of the container 10, the volume of the air 12 in the container 10 increases causing the pressure of the air 12 within the container 10 to decrease. The flow of liquid 11 through the discharge orifice 14 continues until the pressure (Pair) of the air 12 in the container 10 decreases to a value equal to the atmospheric pressure Po minus the product of the density of the liquid times the vertical distance the discharge orifice 14 is below the surface 13 of the liquid 11. When the pressure of the liquid at the container side of the discharge orifice 14 equals the atmospheric pressure Po, the flow of liquid stops as illustrated in FIG. 1c. It should be noted that the volume of the liquid 11 flowing out of the container is not constant since it depends on the level of the liquid 11 in the container 10 as well as the volume of the air 12 in the container 10.
FIGS. 3a, 3b and 3c illustrate three containers 10 having vent tubes of three distinct lengths. Each container 10 also has a small discharge orifice 14. FIG. 3a illustrates a container 10 with a vent tube 31 that is in fluid communication with the atmosphere and the interior of the container 10. FIG. 3a also illustrates the container 10 in an inverted orientation. When liquid is discharged through the discharge orifice 14, the pressure in the container 10 tends to decrease. When the pressure of the liquid at the exit 32 of the vent tube 31 decreases to slightly below atmospheric, air 33 from the atmosphere enters the container 10 through the vent tube 31. This air 33 replaces the liquid that is discharged and the pressure of the liquid in the container remains constant thereafter. The result is that the pressure at the discharge orifice 14 is maintained above atmospheric since it is below the level of exit 32 and the liquid continues to be discharged. This discharge would occur until the container 10 is empty of liquid.
FIG. 3b illustrates a container 10 with a shorter vent tube 34. In this container 10, the pressure must decrease to atmospheric at the exit 35 of the vent tube 34 before air can enter the container 10 and maintain the pressure of the liquid constant. Since the vent tube 34 is shorter, there is less liquid between the exit 35 of the vent tube 34 and the discharge orifice 14. As a result the pressure at the discharge orifice 14 is lower than it was with the longer vent tube 31 and the discharge rate of liquid is lower. However, the discharge of liquid will still continue until the container 10 is empty of liquid.
FIG. 3c illustrates a container 10 with a zero length vent tube (just an orifice 14). In this case the liquid is discharged until the liquid pressure at the orifice 14 decreases to atmospheric. Only a small percent of the liquid contents of the container 10 is discharged.
It would be desirable for a liquid dispenser to have a vent tube that can change its length automatically to decrease the discharge flow rate so that the tube length becomes zero when a desired volume of liquid is discharged and the flow stops.
It would also be desirable to realize this effect without any moving parts.
In certain situations, it would also be desirable to dispense a measured volume of liquid.